This invention relates in general to apparatus for working on sheet material and which includes a vacuum holddown table for securing a portion of a sheet of material in firmly fixed position on the table surface in the working region of a tool which moves relative to the table surface and in working relation to the sheet material spread on the table surface.
Vacuum holddown tables have gained widespread acceptance in the sheet material processing art, however, the equipment used to generate vacuum is usually relatively large, consumes considerable energy and tends to be quite inefficient. In order to increase efficiency of such apparatus, "zoned" vacuum tables have been provided which permit migration of a vacuum zone relative to a supporting surface of a holddown table so that vacuum may be concentrated or applied locally to a portion of the supporting surface in the working region of a movable instrument or tool. On such sheet material processing apparatus, illustrated and described in U.S. Pat. No. 3,495,492 to Gerber et al, issued Feb. 17, 1970, and assigned to the assignee of the present invention, has a vacuum holddown table which utilizes a series of stationary vacuum chambers disposed below a fluid permeable bed which defines the sheet material supporting surface. Valves associated with the chambers operate in response to controlled movement of a tool longitudinally of the table to sequentially connect and disconnect the various chambers to a common vacuum manifold whereby vacuum may be applied to successive zones of the supporting surface. Other apparatus has been provided which includes a vacuum table, a portion of the surface of which is defined by one or more movable belts. Vacuum is applied to an associated portion of the table surface through a gap in the belts. While such apparatus has proven generally satisfactory it includes considerable mechanism and is relatively costly to produce. Further, such apparatus generally applies vacuum to a strip or zone of the table surface which extends transversely across the entire width of the surface. Thus, vacuum is applied to a somewhat greater area of the table surface then may be necessary to effect efficient holddown of the material in the working region of a tool, which results in inefficient utilization of vacuum generating equipment. The present invention is concerned with the aforesaid problems.